1. Field of the Invention
The present invention relates to an oxygen sensor for detecting oxygen in a gas to be measured, such as exhaust gas from an internal combustion engine.
2. Description of the Related Art
A known oxygen sensor includes an oxygen detection element assuming the form of a hollow rod which is closed at a front end, and having electrode layers formed on the inner and outer surfaces thereof. In an oxygen sensor of this type, while the atmosphere serving as a reference gas is introduced into an oxygen detection element such that the inner surface (internal electrode layer) of the element is exposed to the reference gas, the outer surface (external electrode layer) of the oxygen detection element is exposed to exhaust gas. As a result, an electromotive force is induced in the oxygen detection element by the oxygen concentration cell effect, according to the difference in oxygen concentration between the inner and outer surfaces. This electromotive force induced by the oxygen concentration cell effect is led out from the internal and external electrode layers through lead wires and serves as a detection signal indicative of oxygen concentration in the exhaust gas.
In an oxygen sensor of this type, when the temperature of exhaust gas is low, as is the case upon startup of an engine, an oxygen detection element formed of a solid electrolyte member is not sufficiently active, consuming a considerably long time before providing an electromotive force which is sufficiently large to be measurable. In order to cope with this problem, a rodlike heating member having a heating portion is inserted into a hollow portion of the oxygen detection element so as to activate the oxygen detection element through application of heat at the time of startup of the engine, thereby promptly rendering output (electromotive force) available for measurement at the time of startup of the engine, when exhaust gas contains a relatively large amount of harmful components.
In order to efficiently transmit to the oxygen detection element heat generated by the heating member, for improved activity of the oxygen detection element at the time of startup, the oxygen sensor may assume a structure such that the heating portion of the heating member is brought into contact with the inner wall surface of the hollow portion of the oxygen detection element. In such an oxygen sensor, a metallic terminal memberxe2x80x94which is inserted into the hollow portion of the oxygen detection element and is electrically conductive with an internal electrode layer formed on the inner surface of the oxygen detection elementxe2x80x94as a single or a plurality of holder portions (holder means), each having a substantially C-shaped cross section, so as to hold the heating member. Being held by means of the holder portion(s), the heating member is disposed within the oxygen detection element such that a front end portion of the heating member is in contact with the inner wall surface of the hollow portion of the oxygen detection element. In order to hold the heating member firmly and to maintain the end portion of the heating member in contact with the inner wall surface of the hollow portion of the oxygen detection element, the metallic terminal member including the holder portion(s) assumes a complicated form which requires bending in the course of manufacture thereof. Further, assembly of the oxygen sensor requires a jig for preliminarily attaching the heating member to the metallic terminal member. These features increase the cost of manufacture of the oxygen sensor.
It is therefore an object of the present invention to provide a sensor structure which does not require attachment of special heating-member holder means to a metallic terminal member and which allows for simple assembly.
To achieve the above object, an oxygen sensor according to a first aspect of the invention is characterized by comprising:
an oxygen detection element assuming the form of a hollow rod which is closed at a front end, and having electrode layers formed on the inner and outer surfaces of a hollow portion thereof;
a rodlike heating member disposed within the hollow portion of the oxygen detection element and adapted to heat the oxygen detection element; and
a metallic terminal member formed so as to circumferentially surround the heating member and having an attachment portion, which is fixedly attached to the inner surface of the oxygen detection element, directly or indirectly via another member.
The metallic terminal member includes at least one press portion for pressing the heating member in a direction intersecting the center axis of the hollow portion of the oxygen detection element.
The heating member is held by holding means formed separately from the metallic terminal member, and the press portion causes at least a portion of the heating member to be in contact with the inner wall surface of the hollow portion of the oxygen detection element.
According to the first aspect of the invention, the metallic terminal member includes a press portion for pressing the heating member in a direction intersecting the center axis of the hollow portion of the oxygen detection element. Also, the holding means for holding the heating member is formed separately from the metallic terminal member (i.e., the holding means is formed independently of the metallic terminal member). Thus, at least a portion of the heating member can be in contact with the inner wall surface of the hollow portion of the oxygen detection element without the metallic terminal member having special heating-member holder means. Further, assembly of the oxygen sensor does not require a jig for preliminarily attaching the heating member to the metallic terminal member, thereby reducing cost.
The term xe2x80x9ccontactxe2x80x9d typically implies one of the following three kinds of so-called laterally-abutting structure, in which the surface of a heating portion formed at a front end portion of the heating member is laterally pressed against the inner wall surface of the hollow portion of the oxygen detection element.
(1) First it is conceivable that only a front-end portion of the surface of the heating member is in contact with the inner wall surface of the hollow portion (so-called point contact state or near point-contact state). This state of contact arises, for example, when the center axis of the heating member and that of the hollow portion intersect. In the vicinity of the heating portion of the heating member, the center axis of the heating member is laterally biased (offset) from the center axis of the hollow portion of the oxygen detection element.
(2) Next it is conceivable that the surface of the heating portion of the heating member is in contact with the inner wall surface of the hollow portion over a relatively long distance (so-called line contact state or near line-contact state). This state of contact arises, for example, when the center axis of the heating member is substantially in parallel with the center axis of the hollow portion of the oxygen detection element. The center axis of the heating member is laterally biased (offset) from the center axis of the hollow portion of the oxygen detection element.
(3) Further it is conceivable that the surface of the heating member is in contact with the inner wall surface of the hollow portion of the oxygen detection element over substantially the entire length of the heating member (so-called overall contact state or near overall-contact state). This state of contact arises, for example, when the center axis of the heating member approaches that of the hollow portion of the oxygen detection element such that the distance therebetween decreases toward the front-end side. The center axis of the heating member is laterally biased (offset) from the center axis of the hollow portion of the oxygen detection element.
Point contact, line contact, and overall contact are all applicable to the first aspect of the invention.
In the first aspect of the invention, the holding means for holding the heating member is formed separately from the metallic terminal member. Specifically, preferably, the oxygen sensor further comprises:
an external cylindrical member for accommodating the oxygen detection element;
a lead wire connected to the metallic terminal member and adapted to lead out an output from the oxygen detection element to the exterior of the oxygen sensor; and
a grommet having a lead wire through-hole formed therein for passing the lead wire therethrough and fitted into a rear-end opening portion of the external cylindrical member, the grommet filling the space between the inner wall of an opening portion of the external cylindrical member and the lead wire for the sake of seal.
In this preferred oxygen sensor, the holding means is a frictional force induced between the grommet and a portion of the lead wire located within the lead wire through-hole. The grommet and the lead wire are utilized as the holding means for holding the heating member, thereby reducing cost.
To achieve the above object, an oxygen sensor according to a second aspect of the invention is characterized by comprising:
an oxygen detection element assuming the form of a hollow rod which is closed at a front end, and having electrode layers formed on inner and outer surfaces of a hollow portion thereof;
a rodlike heating member disposed within the hollow portion of the oxygen detection element and adapted to heat the oxygen detection element; and
a metallic terminal member formed so as to circumferentially surround the heating member and having an attachment portion, which is fixedly attached to an inner surface of the oxygen detection element, directly or indirectly via another member.
The metallic terminal member includes at least one press portion for pressing the heating member in a direction intersecting the center axis of the hollow portion of the oxygen detection element.
The press portion causes the heating member to extend along and in contact with the inner wall surface of the hollow portion of the oxygen detection element.
According to the second aspect of the invention, the metallic terminal member does not employ special heating-member holder means, but the press portion thereof merely presses the heating member so as to cause direct contact of the heating member with and along the inner wall surface of the hollow portion of the oxygen detection element. Thus, heat generated by the heating member is efficiently transmitted to the oxygen detection element.
In the present invention, when the metallic terminal member includes at least two press portions located apart from each other in an axial direction thereof, the heating member is unlikely to come off the metallic terminal member, which would otherwise occur due to vibration, and the heating member can be stably pressed. Thus, the heating member can be maintained in stable contact with the oxygen detection element.
Preferably, the present invention is such that:
a ceramic separator having a lead wire through-hole formed therein for passing the lead wire therethrough is disposed on the rear-end portion side of the oxygen detection element;
the ceramic separator has a heating-member-end-portion accommodation hole formed therein in such a manner as to extend thereinto from a front end face thereof; and
a bottom surface of the heating-member-end-portion accommodation hole is located at an axially intermediate portion of the ceramic separator and serves as positioning means for the heating member. Thus, the ceramic separator, which is an existing member, can be utilized as positioning means for the rear end portion of the heating member. Further, through insertion of the rear end portion of the heating member into the heating-member-end-portion accommodation hole, the overall length of the oxygen sensor can be reduced, thereby reducing the size of the oxygen sensor.
Preferably, at least one of the press portions is formed on the attachment portion of the present invention. Since a pressing force effected by the press portions is imposed directly on the inner circumferential surface of the attachment portion, which is fixedly attached to the inner surface of the oxygen detection element, the heating member is reliably held while being pressed in a direction intersecting the center axis of the hollow portion of the oxygen detection element. Also, the metallic terminal member is effectively prevented from having any play or coming off.
Preferably, the press portion formed on the attachment portion of the present invention is formed such that at least a portion of the circumferential wall of the attachment portion projects radially inward. A portion of the circumferential wall of the attachment portion is utilized as the press portion, whereby the press portion can reliably press the heating member in a direction intersecting the center axis of the hollow portion of the oxygen detection element. Since the press portion is an integral portion of the metallic terminal member, cost can be reduced.
The press portion of the present invention includes an integrally formed guide segment adapted to effect smooth insertion of at least a front end portion of the heating member when the heating member is to be inserted into the metallic terminal member. By virtue of the guide action of the guide segment, the heating member can be smoothly inserted into the metallic terminal member, thereby improving work efficiency. The guide segment of the press portion may extends in such a direction that the distance from the external circumferential surface of the heating element increases in a continuous or stepwise manner.
According to the present invention, a portion of the circumferential wall of the attachment portion projects toward the external circumferential surface of the heating member so as to form a protrusion portion, and the location of the protrusion portion corresponds to the location of contact between the heating member and the inner wall surface of the hollow portion of the oxygen detection element. This protrusion portion restricts the degree of freedom with respect to the radial movement (the degree of radial play) of the heating member, thereby reducing the radial play of the heating member and suppressing the axial movement of the heating member to a low level.
Preferably, the present invention is such that:
the attachment portion is inserted directly or indirectly via another member into a counter-bore portion which is formed in the oxygen detection element in such a manner as to extend axially over a predetermined length from the end face of a rear-end opening portion of the hollow portion toward a front end portion of the oxygen detection element, the counter-bore portion having a bore diameter greater than that of the hollow portion; and
the position where the protrusion portion presses against the heating member is located on an extension line of the inner wall surface of the hollow portion of the oxygen detection element. Thus, the metallic terminal member does not move radially inward beyond the inner wall surface of the hollow portion. Substantially the entire outer circumferential surface of the attachment portion is in contact with the inner wall surface of the hollow portion, whereby the heating member can be easily disposed so as to extend along the inner wall surface of the hollow portion and can be prevented from having any play.
The protrusion portion of the present invention includes an integrally formed guide segment adapted to effect smooth insertion of at least a front end portion of the heating member when the heating member is to be inserted into the metallic terminal member. By virtue of the guide action of the guide segment, the heating member can be smoothly inserted into the metallic terminal member, thereby improving work efficiency. The guide segment of the protrusion portion may extend in such a direction that the distance from the external circumferential surface of the heating element increases in a continuous or stepwise manner.